**Question 1:**
What is the common difference in the following arithmetic series: $2.5,6.4,10.3,\mathrm{...}$?

**Question 2:**
Which of the following sequences is an arithmetic series?

**Question 3:**
What is the $25$th term in the following arithmetic series: $1,4,7,\mathrm{10...}$?

**Question 4:**
What is the $31$st term in the following arithmetic series: $45,42.5,\mathrm{40...}$?

**Question 5:**
What is the sum of the first $21$ terms in the following arithmetic series:
$\mathrm{3,5,7,9}$?

**Question 6:**
What is the sum of the first $18$ terms in the following series: $\mathrm{21,17,13}$?

**Question 7:**
How many terms in the following arithmetic series
$\mathrm{5,9,13,...}$ must be taken if the sum is $702$?

**Question 8:**
How many terms in the following arithmetic series $0.25,\mathrm{0.75...},$ must be taken if the sum is $156.25$?

**Question 9:**
If the $7$th term is $15$ and the $20$th term is $41$, find the series

**Question 10:**
If the $5$th term is $10$ and the $11$th term is $-14$, find the sum of the first $20$ terms